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Grid Cells: Putting Rats in Their Places and (Maybe) Meaning in Life

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Welcome to Mind Matters. Below, in an enthralling pair of posts about how neural mechanisms of navigation may also underlie memory and cognition, neuroscientists James J. Knierim and A. David Redish provide the first installment of Mind Matters, sciam.com's new expert blog on the sciences of mind and brain. Mind Matters offers something unique: Each week, top researchers in neuroscience, psychiatry, and psychology will discuss recent papers from these disciplines with readers. They will explain their field's most significant new findings -- and discuss what they, as fellow researchers and observers, find most important, exciting, maddening, odd, curious or otherwise noteworthy in the research driving their fields. Mind Matters brings science's most intriguing results and puzzles straight to the public. Blog visitors can observe and participate in the sort of discussion -- informed but informal, relaxed but rigorous -- that scientists share in seminar rooms and corridors at scientific conferences. We hope you'll join us. -- David Dobbs, Editor, Mind Matters
This Week's Paper: "Microstructure of a spatial map in the entorhinal cortex," by Torkel Hafting, Marianne Fyhn, Sturla Molden, May-Britt Moser and Edvard I. Moser, of the Centre for Biology of Mind, Trondheim, Norway; from Nature, 11 August 2005.
Grid cells
Projected order: A representation of the locations that activate one grid cell in a rat's brain. The spots are about 20 inches (50 cm) apart from one another. Illustration by Hafting, Fyhn, Molden, Moser and Moser, used by permission.

Introduction

by David Dobbs
While "Microstructure of a spatial map in the entorhinal cortex" (or "the grid cell paper," as many neuroscientists call it) struck many neuroscientists as an absolute stunner, a layperson can easily find the paper about as accessible as its title. The problem lies in the complex, rolling beauty of the discovery the authors reported. They found in the rat's brain a group of neurons that project across the rat's environment tens of thousands of virtual gridworks, each composed of equilateral triangles. No two of these gridworks are identically sized or placed, and each is tied to a single grid cell. Every time the rat's head passes over one of any grid's many vertices (the spots in the figure above), the associated grid cell fires. The system appears to let the rat constantly track its position in the world. This short description doesn't do the grid-cell system justice -- which is why cognitive scientists James Knierim and A. David Redish explain this seminal paper's findings and implications more fully below. As Knierim's description and Redish's commentary (in following post) suggest, the grid cell system seems to do far more than tell a rat its location; it may well be the key to how the rat -- and we humans, too -- give meaning to memory.

Grid Cells: The Brain's Graph Paper, and Then Some

Description and Commentary by James J. Knierim, Ph.D. University of Texas Medical School, Houston
IN THE 2001 cinematic thriller Memento, the lead character suffers a brain injury that makes him unable to remember events for longer than a minute or so. This type of amnesia is well known to neurologists and neuropsychologists who study patients with damage to the hippocampus, one of the oldest parts of the brain, and a related area of surrounding cortex, the medial temporal lobe. These patients remember events from their life histories that occurred before their injuries but can't form lasting memories of anything that occurs afterward. Their life histories, as far as they recollect, ended shortly before the onset of their disorder. Navigation as memory Exactly how the medial temporal lobe memory system creates and stores these autobiographical memories -- called episodic memory -- has puzzled and fascinated scientists for many years. Clues have come from studies of human amnesics and normal subjects as well as from animal studies. A major advance came in the 1970s, when John O'Keefe and Jonathan Dostrovsky (now at University College London and Toronto Western Research Institute, respectively) discovered that neurons in the hippocampus display place-specific firing. That is, a given "place cell," as O'Keefe dubbed these hippocampal neurons, would briskly fire action potentials (the brief electrical impulses neurons use to communicate) whenever a rat occupied a specific location, but would remain silent elsewhere. Thus each place cell fired for only one location, much as would a burglar alarm tied to a particular tile in a hallway. Similar findings have been subsequently reported in other species, including humans. These remarkable findings led O'Keefe and Lynn Nadel (then at University College London, now at the University of Arizona in Tucson) to propose, in 1978, that the hippocampus was the neural locus of a "cognitive map" of the environment. In their view, the place cells of the hippocampus performed a crucial neurological and cognitive function, organizing the various aspects of experience within the framework of the locations and contexts in which the events occurred. This contextual framework allowed the relationships between an event's different aspects to be stored in a way that allowed later retrieval from memory. Over the years, this particular view has been hotly debated. Yet a consensus is emerging that the hippocampus does somehow provide a spatial context that is vital to episodic memory. When you remember a past event, you remember not only the people, objects, and other discrete components of the event but also the spatiotemporal context in which the event occurred, allowing you to distinguish this event from similar episodes with similar components. Despite intensive study, however, the precise mechanisms by which the hippocampus creates this contextual representation of memory have eluded scientists. A primary impediment was that we knew little about the brain areas that feed the hippocampus most of its information. Early work suggested that the entorhinal cortex, an area of cortex next to the hippocampus, contained neurons that encoded space similarly to the hippocampus, but with less precision. "This changes everything." This view has now been turned completely upside down with the amazing discovery, described in the paper at hand, of a system of "grid cells" in the medial entorhinal cortex. Unlike a place cell, which typically fires when a rat occupies a single, particular location in an environment, each grid cell will fire when the rat is in any one of a number of locations that are arranged in a stunningly regular hexagonal grid -- somewhat as if the cell were tied to a number of alarm tiles spaced at specific, regular distances. The locations that fire each grid cell are arranged in a precise, repeating grid pattern composed of equilateral triangles that tessellate the floor of the environment. (See illustration above.) Imagine arranging dozens of round dinner plates to cover a floor in their optimal packing density, such that every plate is surrounded by six other, equidistant plates; this arrangement mimics the triggering pattern tied to any given grid cell. As the rat moves around the floor, a grid cell in its brain fires each time the rat steps near the center of a plate. Other grid cells, meanwhile, are associated with their own hexagonal gridworks, which overlap each other. Grids of neighboring cells are of similar dimensions but are slightly offset from each other. These grid cells, conclude Hafting and colleagues, are likely to be key components of a brain mechanism that constantly updates the rat's sense of its location, even in the absence of external sensory input. And they constitute the basic spatial input that allows the hippocampus to create the highly specific, context-dependent firing of its place cells. This discovery is one of the most remarkable findings in the history of single-unit recordings of brain activity. I remember vividly the sense of excitement I felt when I read this paper in my office for the first time. I realized immediately that I was reading a work of historic importance in neuroscience. No one had ever reported a neural response property that was so geometrically regular, so crystalline, so perfect. How could this even be possible? Yet the data were convincing. "This changes everything," I muttered. My excitement rose partly from the sheer beauty of the grid-cell response pattern. But it rose too from a belief that this was a major step in our quest to understand how the hippocampus might form the basis of episodic memory. Grid cells give us a firm handle on what kind of information is encoded in one of the major inputs into the hippocampus. From this we can start to create more realistic models of what computations occur in the hippocampus to transform these grid representations into the more complex properties that have been discovered about place cells over the past three decades. For example, different subsets of place cells are active in different environments, whereas all grid cells appear to be active in all environments. How is the general spatial map encoded by grid cells turned into the environment-specific (or context-specific) maps encoded by place cells? Moreover, the discovery of grid cells affirms emphatically that the hippocampus and medial temporal lobe are outstanding model systems for understanding how the brain constructs cognitive representations of the world "out there" that are not explicitly tied to any sensory stimulation. There is no pattern of visual landmarks, auditory cues, somatosensory input, or other sensations that could possibly cause a grid cell to fire in such a crystalline fashion throughout any environment. This firing pattern -- which is the same regardless of whether the rat is in a familiar lit room or in a strange place that's pitch-dark -- must be a pure cognitive construct. While doubtless updated and calibrated by sensory input from the vestibular, visual, and other sensory systems, grid-cell firing patterns do not depend on such external sensory cues. Some have argued that hippocampal place cells were similarly independent, but the known influence of external landmarks on these cells, and their tendency to fire in single locations, led others to argue that place cells were primarily driven by unique combinations of sensory landmarks that occurred at particular locations. This argument cannot explain the firing patterns of grid cells. A matter of context What, then, does account for grid-cell dynamics? One possibility is that that these cells are where the animal uses information about its own movement through the environment to update its location on its internal, "cognitive map" -- where it translates information about recent, small changes in location into a sense of where it stands and where it's going in the larger world. The hippocampus, in turn, may be the brain structure that combines this spatial representation with other information about the discrete items that make up an event -- and thereby creates memories of unique experiences in particular spatiotemporal contexts. This ability is exactly what the protagonist in Memento lost. The discovery of grid cells has generated a palpable sense of excitement -- an anticipation that further research into grid cells and into the other major input to the hippocampus, the lateral entorhinal cortex, will reveal the neural mechanisms that let us remember and make sense of our personal histories: a vital process that forms the very foundation of one's sense of identity.
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CLICK HERE to read A. David Redish's accompanying commentary on grid cells, "Through the Grid, a Window on Cognition."
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James J. Knierim, Ph.D., is an associate professor of neurobiology and anatomy at the University of Texas Medical School at Houston, where his lab studies the role of the hippocampus and related structures in spatial learning and memory.

The views expressed are those of the author and are not necessarily those of Scientific American.

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